Conventionally, there is known an observation plug for observing a combustion chamber. For example, Patent Document 1 discloses an ignition plug having incorporated therein an image capturing device that captures an image of the inside of the combustion chamber through a visualization window. Also, Patent Document 2 discloses an in-cylinder observation device including a visualization plug provided with a lens portion exposed into a combustion chamber, and a camera provided to the lens portion via a bore scope portion. The in-cylinder observation device captures an image of the inside of the combustion chamber with the camera arranged outside the internal combustion engine.